Message-ID: <4C18D420.8090008@kernel.org>
Date: Wed, 16 Jun 2010 15:39:44 +0200
From: Tejun Heo <tj@kernel.org>
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To: Johannes Berg <johannes@sipsolutions.net>
CC: mingo@elte.hu, awalls@radix.net, linux-kernel@vger.kernel.org,
       jeff@garzik.org, akpm@linux-foundation.org, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com,
       oleg@redhat.com, axboe@kernel.dk
Subject: Re: Overview of concurrency managed workqueue
References: <1276551467-21246-1-git-send-email-tj@kernel.org>	 <4C17C598.7070303@kernel.org> <1276695468.3640.19.camel@jlt3.sipsolutions.net>
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On 06/16/2010 03:37 PM, Johannes Berg wrote:
>> As multiple execution contexts are available for each wq, deadlocks
>> around execution contexts is much harder to create.  The default
>> workqueue, system_wq, has maximum concurrency level of 256 and unless
>> there is a use case which can result in a dependency loop involving
>> more than 254 workers, it won't deadlock.
> 
> I see a lot of stuff about the current limitations etc., but nothing
> about code that actually _relies_ on the synchronisation properties of
> the current wqs. We talked about that a long time ago, is it still
> guaranteed that a single-threaded wq will serialise all work put onto
> it? It needs to be, but I don't see you explicitly mentioning it.

Oh yeah, if you have WQ_SINGLE_CPU + max inflight of 1, works on the
wq are fully ordered.

-- 
tejun
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